re: How does buying a phone off ebay work with my carrier?

I have a similar issue. Seller says:

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I just went into my Google Project Fi account and while this phone is showing deactivated, it's still showing as registered to my account.  Two weeks ago I requested Google to charge my credit card the balance to pay the phone off and remove it from my account.  Unbeknownst to me, the payoff isn't immediate.  Instead it happens on the billing cycle date.  Well, my date is next Tuesday, 11/22.  It looks like the phone will reach you on Monday, so it's not going to be able to be activated until Tuesday

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I then ask about just putting my sim in it because I don't understand. He says,

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The way I understand it is the phone EIN has to be released from the current provider before a new provider activates the phone.

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Can someone explain this to me? May be moot since the phone did not come in today.